Unbelievable that this hasn't been addressed.  This is a MAJOR issue.  The work around of adding a $0 item to the PO to keep it open to post the bill against works, but that leaves the item receipts open and can increase the risk that a vendor bill could be entered twice.  This issue makes the PO related transaction report completely unusable as the bills are no longer attached to the PO.  Also the PO transaction screen shows 0 items on bills.  I regret the upgrade to 2021.